Conventionally, a variable-focus optical system that is disposed between an objective lens and an imaging lens in a microscope apparatus and that varies the working distance of the objective lens has been known (for example, refer to PTL 1). The variable-focus optical system described in PTL 1 includes a front-group optical system disposed on the objective lens side and a rear-group optical system disposed on the imaging lens side, wherein, by disposing the front focus of the front-group optical system near the rear focus of the objective lens and driving at least one of the front-group optical system and the rear-group optical system in the optical axis direction, variations in the observation magnification can be suppressed while allowing the working distance of the objective lens to be changed.